ministerie van verkeer en waterstaat rijkswaterstaat

dienst weg- en waterbouwkunde afdeling advisering waterbouw coastal measurements in the Netherlands

Jarkus

Introduction Because a sandy coast is a very dynamic coastline, and because stability of the coastline is vital for the safety of the Netherlands against inundation by storm surges (dunes protect the polder-area, which is situated below sea le­ vel), already in former centuries it was necessary to get some knowledge about the movements of the coastline. In the middle of the 19th century systematic coastal measurements started. On fixed intervals (approx. 250 m) the position of the dune-foot, the high water line and the low water line was measured every year. In order to have a fixed reference, monuments were placed on the beach. Around 1960 it became clear that more detailed information was necessary. Therefore the system of monuments and base lines was somewhat updated, and a detailed program for coastal measurements was set up. Along the whole North Sea coastline from Cadzand in the south to Rottumeroog a fixed set of measuring lines was defined (see fig. 1). In total approx. 3000 lines are defined this way. The lines have a intermediate distance of 200 m and are perpendicular to the base line. The base line is parallel to the coast­ line, and is positioned approx. near the high water line. On coastal sections with groins, the measuring lines are placed in the middle between the groins. Profiles are measured every year from 200 m landward of the base line to 800 m seaward of the base line (that means to an average depth of 8 m below mean sea level). Every five years every kilometer a line is measured until 2500 m from the base line. All measurements are made in the period between april and september. , The measurements are performed in two parts (see fig. 2): * leveling above the low wat.er line * sounding below approx. mean sea level line leveling The levelings are done by the survey department of Rijkswaterstaat, using aerial photography (see fig. 3). In the field only some calibration points are measured using conventional equipment. In flight direction 60 % overlap is used in order to get a good stereoscopic effect. In more than one strip is required, an overlap of 20 - 30 % between the strips is used. With the use of an analytic plotter the height is measured in the measuring lines. In this plotter the overlapping photographs are presented a a three dimensional image. The position of the bends in the profile are read as digital values. The program interpo­ lated the data, and every 5 m .a height-point of the profile is stored in memory. soundings The soundings are done by survey vessels of various Rijkswaterstaat departments and by survey vessels of the waterboards. Profile data are automatically stored on board as x-y-z- values. Postprocessing equipment calculates the depths in the profile with intermediate distances of 10 m.

1 storage Leveling data and sounding data are coupled and stored on a mainframe computer of Rijkswaterstaat. The data are available to coastal managers, researchers and everyone else who is interested. All data are on-line available since 1963. This data-base is called Jarkus (abbreviation from JAaRlijkse KUStmetingen, yearly coastal measurements).

Post processing software is available to read the data, and to perform additi­ onal computations. Standard Postprocessing is plotting profiles (fig. 4) and making three dimensional plots (fig.5). Also volumetric calculations can be made. This can be done in horizontal and in vertical slices (fig. 6). The vertical sections are used for gentle coastlines, while the horizontal sections are used for coastlines facing relatively steep tidal channels.

Records of the Jarkus-database have a fixed format (fig. 7). This format has become the standard of all coastal information systems and coastal computer models in the Netherlands.

One measurement consists of a header and depth data. The header format is 716 and contains the following information: 1. Coastal section 1. Rottum 9. Delfland 2. Schiermonnikoog 10. Maasvlakte 3. Ameland 11. Voorne 4. Terschelling 12. Goeree 5. Vlieland 13. Schouwen 6. Texel 14. Noord Beveland 7. Noord Holland 15. Walcheren 8. Rijnland 16. Zeeuwsch-Vlaanderen 2. year, in 4 digit (1982) 3. profile number (which is in fact a distance from a zero-point along the coast). The profile number is given in 10 m (thus: profile 07 - 234 is 2.34 km south of Den-Helder along the coast of Noord Holland). 4. Profile type 0 - normal (official) profile 1 - additional profile in the axis of a groin 2 - additional profile parallel to a groin 5. date of measurement (leveling) ddmm 6. date of sounding ddmm 7. number of points The depth lines contain the following information 5(2X6,11,3X) 8. horizontal distance in meters from base line (seaward is positive) 9. vertical distance in centimeters relative to National Datum (NAP), which is approx. at Mean Sea Level (depth are negative, heights are positive). 10. control code (from the code it can be seen if data are leveling or soun­ ding data).

Dune erosion

The Jarkus database is at this moment in use for routine checks of the safety of our dune coast. Every year all profiles have to be judged if the provide enough safety. The technique to do this is described in the "guide to the assessment of the safety of dunes as a sea defense" [english edition, CUR, 1989]. In fact an storm induced erosion analysis has to be made for a series of pro-

2 Location (10^ tPy period

1 toaland 2,2 1980 2 Amelarri 0,31 1979 3 Texel, Eierlarei 3,05 1979 4 Texel, Eierland ' 2,85 1985 5 Texel, da Koog 3,02 1984 6 Callantsoog 0,35 1976-1977 7 Callantsocg 0,47 1979-1980 8 Scheveningen 0,045 1969 9 Scheveningen 0,7 1975 10 Scheveningen 0,33 1985 11a Heek V. Holland 18,94 1971-1972 11b Hoek V. Holland 1,50 1976-1977 12 Hoek V. Holland Htwil van Ho&aR3 0,87 1977-1978 13 Brielsa Gat Ceua 0,15 1979-1980 14 Voorra 0,11 1974 15 Voorna 0,15 1974 16 Voorna 1,10 1977 ©©©©©(5> 17 Voorna 0,44 1983 18 Voorna 3,40 1984-1985 19 Goeroa 0,40 1969-1970 20 Goeree 0,61 1971 21 Goeree 3,64 1973-1974 22 Goeree 1,27 1977 © 23 Goeree 0,33 1984 24 Goeree 0,53 1985 25 Schouwen 0,112 1975 26 Noord Beveland 0,21 1973 27 VÈuLcheren 0,10 1984 28 Vfalcheren 0,775 1952-1959 (i])®(ü) 29 vlissingen 0,05 1952 30 Vlissingen 0,032 1966 31 Vlissingen 0,045 ,1975 32 Breskens 0,206 1971 33 Callantsoog O GanI 1,3 1986 34 Texheijde 3,0 1986 35 Voome 1,0 1986 36 Conburg 0,23 1986 37 Kop V. Schouwen 1,83 1987 38 Westkapelle 0,025 1988 39 Westkapelle 0,41 1988 40 Zoutelande 0,15 1988 41 Cadzand 0,85 1988 42 Zwanenwater 1,85 1987 43 Hoek V. Hollard 0,2 1988

Totaal 59,14 file measurements (approx. 10 years). For each year the calculation resulted in a maximum erosion point during a design storm. Through the calculated erosion points (point P), a regression line has to be drawn (fig. 8). Extrapolation of this line until next year indicates if there is still sufficient safety availa­ ble. Computer programs on a PC are available to perform the computations, using the data from the Jarkus database. All coastal managers have to do these computa­ tions on a routine basis (fig. 9).

Evaluation of beach nourishment projects

In October 1987 a report has been prepared to evaluate beach nourishment pro­ jects in the Netherlands. For the preparation of the Coastal Memorandiim of 1989 a number of technical reports has been prepared. The technical report nr. 12 (beach and dune nourishment) is a follow up of the report to the parliament of October 1987. In this paper a summary of these reports is given, with some more detailed information on a few specific projects.

Beach nourishments can be placed at several sites in the cross-sectional pro­ file, see figure 10. Sand is in the Netherlands almost always placed by hydrau­ lic equipment. Dry hauling is hardly used. Using hydraulic dredges a quantity of 200 000 - 500 000 m-^ per week is feasible. The coast of a nourishment work increase significantly if the sand has to be placed higher on the beach. In figure 11 is indicated the quantity of sand nourished on the Dutch coast in the former years. Table 1 gives an overview of the executed nourishment works.

For the evaluation a selection was made from all the projects. The criteria for this selection were - the size of the nourishment, in relation to the natural process of erosion - the type of nourishment; dune improvements were not evaluated. The evaluation is done by using the data from the Jarkus - database. It is assumed that the boundary of the active zone lies within 800 m from the base lines and that consequently the whole active zone is covered by the Jarkus data. Because of tidal channels for the nourishments in Zeeland data are only used until the middle of the tidal channel. On the basis of the data in the Jarkus database the volume of the coastal section in front of the nourishment has been calculated. This has been done for a number of years, before and after the nourishment. In this way it is possible to determine how much sand is disappearing due to the natural erosion of the coast and of the effect of the nourishment. In the first year generally sand moves quite fast from the dry beach to the region around the low water line. The visual effect is a quick decrease of the beach. Volumetric computation show, however, that not really very much sand disappears from the profile. The adaptation to the natural profile is complete­ ly filtered out, by making correct volumetric computations. Nine projects were evaluated more in detail to see if the nourishments did fulfill the expectations. Table 2 summarizes these projects:

3 proj ect year design life expected lifetime 1. Ameland 1980 10 years 1990 will be reached 2. Texel-north 1979 5 years fulfilled expectations 3. Texel-north 1985 5 years had a lifetime of 4 years 5. Texel-middle 1984 10 years probably 1994 will be reached 9. Scheveningen 1975 nourishment only if necessary 16. Voorne 1977 5 years had a lifetime of more than 5 years 21. Goeree 1973 - had a lifetime of more than 5 years 22. Goeree 1977 5 years had a lifetime of more than 5 years 27. Westkapelle 1984 5 years fulfilled expectations Table 2: results of beach nourishment evaluation

From the evaluation followed that: * From the 9 evaluated nourishments, 5 did fulfill the expectations, three will probably fulfill the expectations and one failed (lifetime 4 years instead of 5 years). * It has been proved that it is possible within the varying situations along a natural coastline, to design a beach nourishment project with a pre-defined lifetime. * The application of beach nourishment projects as a method of coastal de­ fense is efficient. The occurring erosion is adequately compensated by the nourished sand, at a relatively low cost. In general all beach nourishments in the Netherlands have been designed on basis of the known coastal recession. Calculated is the volume of sediment which dissappeared during the last years from the coast (i.e. from the top of the dune to a distance of approx. 800 m out of the base line) . Then a quantity of sand was placed on the beach, compensating the erosion during a given number of years. Fron economical optimaztion follows that the optimal design life of a beach nourishment in the Netherlands is generally in the order of 5 years. Every year the volume of sand in the coastal section is determined and plotted. If this quantity becomes lower than a minimum quantity, renourishment is neces­ sary. See also fig. 12. Because the new coastline forms some kind of a promontory into the sea, in the first years some extra erosion might be expected, especially if the length of the nourishment is relatively small. No detailed information is available on the required extra quantity. Mostly an overfill ratio of 20 % is used for this purpose. It is assumed that this overfill ratio also compensates minor diffe­ rences in grain size.

The costs of beach nourishment The costs did vary very much with the borrowing site of the sand for the nourishment. Besides the borrowing area, also the way of determining the price is of importance. Three scenarios are used to determine the cost of sand win­ ning. 1 Present borrow areas more near to the coastline 2 Alternative borrowing sites, based on newest morphological knowledge 3 Present borrow areas, using a high calculation value In table 3 can be seen that the price (including all taxes) varies from US $ 1.50 per m"^, if sand can be borrowed form areas near the coast, up to US $ 5.35, if the borrow area is far away (at a depth of 20 m, 20 km offshore) and a high calculation value is used.

4 scenario for sand winning wadden coast of coast of price in US $ per m area Holland Zeeland

present borrow areas 1.50 4.80 2.40 alternative borrow areas 2.25 2.40 2.40 present borrow areas and 2.90 5.35 3.20 a high calculation value Table 3: price of sand

o Dune nourishment is US $ 2.00 per m more expensive. In determining these prices market effects are not taken into account. Because of overproduction the dredging prices on the world market are lower at this moment, but for basis of a long term policy, this should not be taken into account. beach nourishment on the island of Texel The northern sectiion of the island of Texel has been norished twice, in 1979 and in 1985. Figure 13 shows the effects of the nourisment on the volume of sand. It can be seen that after the nourishment of 1979 the erosion rate was almost identical to the erosion rate befor 1979. beach nourishment on the island of Goereee Figure 14 shows the nourisments on the island of Geree. This is on a first view a strange nourishment, because it is on an accreting coast. The reason of this nourisment was that the dunes did not give enough safety against inundation of the polders behind the dunes, and more sand was required on the dunef ront and the (dry) beach. This sand was placed there in 1974, 1978 and 1985. The beach nourisments caused that too much sand came in the active zone, and the surplus of sand eroded away. beach nourishment near Westkapelle The former island of Walcheren (nowadays it is connected to the mainland by dams and reclamations) is situated in the south-western part of the Netherlands (see fig. 15) . Very near to the south-west coast of the island, a deep and relatively narrow tidal channel, can be found, the Oostgat. Because of the general morphological dynamics in the area (it is the mouth of the river Scheldt) this channel tends to move in a south-eastern direction, and conse­ quently eroding the coast. This is a rather slow process, because boulder-clay layers and other densely packed formulations prevent the erosion. Also, along this coastline a number of groins has been build, each with a groin-head with stone-protection extending to a great depth. These stony "headlands" prevent movement of the channel. Because of these processes and man's reaction on it, the coast is very steep and the beach relatively narrow. The orientation of the coast causes that all waves come from the same direction, consequently there is a clear wave-induced longshore transport from the village of Westkapelle to­ wards the town of Vlissingen in the south of Walcheren. The existing groins (low stone-groins with piles (see fig. 16) do not stop the longshore transport. Almost the entire coast is a sandy coast with a single row of dunes. However near Westkapelle no dunes are present and a heavy sea dike has been construc­ ted. Because of this configuration, there is no natural nourishment for the longshore transport near Westkapelle and consequently the dunes just south of Westkapelle suffer from erosion. In. the past this was the reason to extend the seadike in a more southern direction. Because the polder-area behind the dunes is lower than Mean High Water, the dunes also have a function as seadefense. A breakthrough will cause inundation of the island (In fact in 1944 allied bom­ bers destroyed the dike at Westkapelle in order the inundate the island for

5 strategic reasons. After several raids the succeeded and during the winter 1944/1945 the island was drowned). Figure 16 shows a profile of the coast just south of Westkapelle and it can be seen easily that the remaining dune does not give much safety. In such cases, according to Dutch Laws, reinforcement works have to be per­ formed. Because it was the intention not to extend the seadike any more, rein­ forcement had to be made with sand. In the past (in 1974) a reinforcement was already made on landward side of the dune. Such a reinforcement does provide the required level of safety but does not stop coastal erosion and coastal regression.

Planning the nourishment Therefore the idea was to make future reinforcements by placing sand on the seaward side of the dunes. Thus artificial beach and dune nourishment. A tech­ nical problem which arose was that the beach is very narrow, not allowing placing much sand on the beach. It was decided to place the sand as much as possible against the existing sea front of the dunes. The height of the dune nourishment was equal to the height of the existing dunes. The slope of the beach was determined by the natural slope of free flowing sand from a dis­ charge-pipe in in the surf zone. The amount of sand placed in this way on the coast should guarantee safety for a period of five years. It is in fact a re­ latively small artificial beach nourishment project but this project has seve­ ral interesting aspects from coastal engineering viewpoint. Therefore it was decided to follow this nourishment in detail with measurements of the profile. The nourishment was placed in 1984. The expected life-time was five years, in 1988 a renourishment has been made. This paper describes the changes in the period 1984-1988.

Nourishment quantity The total loss of stand in the period 1971-1981 between km 22.550 and km 23.865 in a strip between dune-foot (MSL - 4.00 m) was 88000 m . Based upon this figure the required quantity for a five years nourishment was 44000 m . Using this quantity, it was possible to improve the dune at the seaward side in order to reach the required safety-level. The width of the dune was therefore in­ creased with 10 m, requiring 22000m^. For losses during execution a surcharge of 10-15% is required. It was decided to place a quantity of 75.000 m on the beach. During the execution of the work the coastal manager decided to place an additional 25000 m^ on the beach, in order to improve two adjacent beach sections and in order to get a better beach level near the stone groins.

Groins Lee erosion, caused by the stone groins, gives a lower beach and a local back­ wards movement of the dunes, just south-east of each groin. In order to main­ tain a higher beach level near to weak dune row at km 23.250, preventing a backwards movement of the dune at that point, the distance between the stone groins was decreased by constructing three extra groins between the existing ones. At the location of the new build stone groins only pile-rows with a stone head were present. These constructions stopped the landward movement of the tidal channel (because of their stone head), but had hardly any influence on erosion caused by surf- included transport. It is expected that the new stone groins have a positive effect on the beach level.

Analysis In order to evaluate the beach nourishment, the regular yearly coastal measure­ ments were used. Along the Dutch coast every year a full profile is measured.

6 approx. 200 m apart. Because of the position of the groins, in this area profiles are measured 125 m apart. See fig. 15. This date are worked out in the following way.

For nine profiles, a yearly volume integration has been made for the period 1973 until 1988. This integration is done in horizontal layers of 1 m thick­ ness, starting form the bottom of the channel until the top of the dune. - three strips have been regarded (see fig. 17). - channel-slope between bottom and MSL - 4.00 m - the "beach" between the channel edge (MSL - 4.00 m) and the dune foot (MSL +3.50 m) - the dune above the dune foot (MSL = 3.50 m). The results of the volume integration is presented in fig. 18, using 1973 as the base-year. In figure 19 the changes in the cross-sectional profile are plotted, using the profile of 1984 (before nourishment) as base line. In this figure also the expected 1987 profile is presented, if no beach nourishment was performed. From figure 195 can be seen that in the period 1984-1987 the expected "natural" ero­ sion is in the order of 4.000 m /m height of profile in the zone between MSL - 4.00 and MSL + 7.00 m. The beach nourishment was mainly placed between MSL and MSL + 2.00 m (17000 m-^/m). In the strip between MSL + 8.00 m the placed quan­ tity was approx. 5000 m /m. In the strip between MSL and MSL - 2.00 m the placed quantity was approx. 6000 m /m.

Observations

From the analysis the following observations can be drawn:

dune 1. The regression-speed of the dune was not changed by the nourishment. The regression-speed is now equal to the speed before nourishment. Three years after placing the sand 70% of the nourished sand is till in the dune- strip. beach 2. Also for the strip "beach", it can be seen that during the first two years no sand was moved out of this strip. In the third year erosion started. In 1987 22% of the placed quantity was transported out of this strip.

3. The beach area between MSL and the dune foot (this is an important value for beach recreation) was increased by d9% due to the nourishment. How­ ever, because of the offshore transport the beach area increased until 23% more of the original surface in the 2nd year. In the 3rd year the beach surface was still 19% more than before nourishment.

dune and heach 4. In the total profile there was not any loss in the first and second year after nourishment. In the third year was a loss of 24%.

5. There was some longshore redistribution of the beach sand, but this quan­ tity was negligible wit respect to the offshore transport.

6. Because of the nourishment a none-natural, steep beach slope was formed. This caused a offshore transport, introducing erosion of the dune front. Approx. 9000 m was transported from the dune towards the beach.

7 As mentioned before, after three years 25% of the material was removed from the section (above MSL - 4.00 m) . It may be assumed that this is mainly caused by offshore transport. The accuracy of soundings in the tidal channel is too low for proving this. The quantity of sand nourished above MSL gives a direct contribution to the safety of the dunes. During the first 3 years 53% of the sand nourished above MSL was still above MSL and contributed to the safety.

Discussion In this case there was not much space for a beach nourishment. This gave the need to place the sand very high on the beach and near the dune front. The resulting beach profile is too steep and not stable. From the data presented follows that the steep profile will adapt to a more stable profile, but that this adaptation is a s low process, In a case like this it is doubtful if even a stable profile will be reached. However, having an unstable, too steep profile gives some advantages. Because relative much sand was placed above MSL the dune became more safe. Also the beach is wider than it would be at a stable beach. It is not known how long the adaptation period is, but this period is in any case longer than five years, which is the life time of the nourishment. So during the whole life-time the profile is unstable, giving the advantages of extra safety and a wider beach.

Conclusion Designing a beach nourishment, the main design quantity is the required quan­ tity of sand. A secondary problem is where to place the sand in the profile. It is advantageous to place the sand as high as possible on the beaoh. Trying to make a stable profile is in many cases not useful.

8

——time (year) 1960 1970 1980

SEASIDE d*g= distance over whicti the regression line is shifted landwards so as to include : - the processing of profile fluctuations {d)

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AFSLAGHOEVEELHEDEN Aanzanding (excl. invloed langstransport) 482.49 (m3) Afslag (excl. invloed langstransport) -482.49 (m3) Afslag boven rekenpeil(excl. invloed langstr. ) -389.88 (m3) Toeslag op afslag boven rekenpeil -117.47 (m3)

AFSTANDEN Aanzandafstand (XCA3-XCB3) 291.86 (m) Afsiagafstand (XCB3-X[E]) 74.33 (m) Extraverschuifafstand tgv langstransport (in BC) ü. 00 (m) Verschuifafstand incl langstransport (X[B3-XCC]) 70. 25 (m) Verschuifafstand voor de toeslag (XCC]-X[D]) 33, 56 (m) Verschuifafstand incl langstransport + toeslag (XCB]-X[D]) 103.82 (m) Xcoord. van afslagprof iel op rekenpeil (XCC]) •103.05 (m) Xcoord. afslag+toeslagprofiel op rekenpeil (XCOj) •136.62 (m)

Oefening Zeeburgerbad alle raaien stormvloed

RWS-DWW DUINAFSLAGPGM DUINAF-PCOOl

3 Location quq!?i:i' iy (10^ bP)' periodi

1 toeland 2,2 1980 2 flnwland 0,31 1979 3 Texel, ElerlciM 3,05 1979 4 Texel, Elerlarxl' 2,85 1985 5 Texel, de Koog 3,02 1984 6 Callantsoog 0,35 1976-1977 7 Callantsoog 0,47 1979-1980 8 Schevenii^en 0,045 1969 9 Schevenin^in 0,7 1975 10 ScheveniiKjen 0,33 1985 11a Hoek V. Holland 18,94 1971-1972 11b Hoek V. Holland 1,50 1976-1977 »o9k van HoUrS 12 HoeJ^ V. Holland 0,87 1977-1978 13 Brielse Gat Dam 0,15 1979-1980 14 Voorr» 0,11 1974 15 Voorna 0,15 1974 16 Voorna 1,10 1977 ©©©©©© 17 Voorna 0,44 1983 18 Voorna 3,40 1984-1985 19 Goerea 0,40 1969-1970 20 Goeree 0,61 1971 21 Goeree 3,64 1973-1974 22 Goeree 1,27 1977 © 23 Goeree 0,33 1984 24 Goeree 0,53 1985 25 Sciiouwen 0,112 1975 26 Noord Beveland 0,21 1973 27 Walcheren 0,10 1984 28 Sfelcheren 0,775 1952-1959 29 Vlissingen 0,05 1952 30 Vlissingen 0,032 1966 31 Vlissingen 0,045 ,1975 32 Breskens 0,206 1971 33 Callantsoog oQsnt 1,3 1986 34 Terheijda 3,0 1986 35 Voome 1,0 1986 36 Dacburg 0,23 1986 37 Kop V. Schouwen 1,83 1987 38 Westkapelle 0,025 1988 39 Westkapelle 0,41 1988 40 Zoutelande 0,15 1988 41 Cadzand 0,85 1988 42 Zwanenwater 1,85 1987 43 Hoek V. Holland 0,2 1988

Totaal 59,14

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.FSLAGHOEVEELHEDEN aanzanding (excl. invloed langstransport) 482.49 (m3) ,fsiag (exci. invloed langstransport) •482.49 (m3) ,fslag boven rekenpei1(eXci. invloed langstr.) -389.88 (m3) oesiag op afslag boven rekenpeil •117.47 (m3)

FSTANDEN anzandafstand (XCAl-XCB]) 291.86 (m) f s iagaf stand (XCB]-XCE]) 74.33 (m) xtraverschuifafstand tgv langstransport (in BC) Ü.00 (m) erschuifafstand incl, langstransport (X[B]-X[C]) 70.25 (m) erschuifafstand voor de toeslag (XCC]-X[D]) 33.56 (m) erschuifafstand incl, langstransport + toeslag (XCB3-XCD]) 103.82 (m) coord, van afslagprofiei op rekenpeil (X[C]) •103.05 (m) coord, afsiag+toeslagprofiel op rekenpeil (XCD]) -136.62 (m) efening Zeeburgerbad aiie raaien stormvloed

US-DWW DUINAFSLAGPGM DUINAF-PCOGl erosion

erosion profile shifts in landward direction until erosion = sand deposit

Fig. 1. Principle ofthe computational model for dune erosion.

— I'= 0.4714 .— —— ,v + 18 2.00 11J \ 0.0268/

A = calculated amount of dune erosion above computational level T= surcharge on A for - duration or srorm surge In 171 - gust surges and gust oscillations J ^^•'•'•1 inaccuracy computational modej

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1965 1967 1959 1971 1973 1983 •. 1985 JAAR yecir __l , I I 1 : i • L-

-ÜOO-

•800-J RAAIVAK < 25^0 - 3081

-12004

•1600-1

STROOK I 100 - 200

STROOK ' 100 - 400 in meters,zeewaarts 600 STROOK 1 100 - t.o.v. de RiJI

WALCHEREN JO0S5ESWEG RAAI 22.937 - 23.685

1 1 1 1 1 1— • 1 1 • 1 AANIAN0IN6 1 VN. \ tov 1«J \.' ^ IN H» _N N. JO.OOOj s

»^ —^ r.. h. ti .... ro.v. 19P 's k N IN H» * és '0 - < —\ a ^ j * crosion \ r«l«biv* to 1925 - - i \ - - - -

\ i\ - -

j : \ lto.«oo k •r "11— - J 1 • «90 fclinC

TUD SUPPLITIÊ CUMELATIEF INHOüDSVERLOOP ZT.r": pT.'^^ot;2"/.^sVT STRAND • PUIN ( NA OOm/TOP PUIN)

Fig, Inhoudsveranderingen sedert 1973. ƒ.3 '5

erosion in accret/on i« »i rr,> ^ Verdrepinq in m"* Aonzandinj In m fCOO 1200 8000 ^000 O

Fig. '."inhoudsverdeUn^ tassen >i.Qf! - 12,00m cn/op duin (vak 22.937

Volt-'mc cfjOogeS fcetwe^^ 'n^l-H <-eKpecked pojit'of wt'è^wot i.3 15